Conventionally, a mobile phone has a handover function for providing a user carrying the mobile phone with stable communication quality during the user moving from one place to another. Specifically, “handover” represents a technique which provides a user with stable communication quality by enabling, when a user performing communication by using a mobile phone in the same wireless communication network moves from a wireless zone of one base station to a wireless zone of another base station, the communication to be transferred smoothly in a relatively short processing time from one base station under connection to another base station to be connected.
As a result of recent progresses of wireless communication techniques, it is now possible that one wireless communication terminal has plural wireless communication devices connectable to different wireless communication networks. In a typical example of such a structure, PDA (Personal Digital Assistance), PC (Personal Computer) or a mobile phone has a first wireless communication device to be connected with a mobile phone network and a second wireless communication device to be connected with a wireless LAN station. In the handover technology, although development of techniques for allowing a mobile phone to move between base stations in a conventional single wireless communication network is in progress, handover technologies based on an environment across plural networks of different types, in a case where plural wireless communication networks of different types are present, have not been well developed yet.
Examples of handover to the same wireless network include handover between areas or sectors of a wide-area wireless network system. When a terminal moves between areas or sectors of a wide-area wireless network system, handover is normally done with respect to a base station located in a better area or sector in terms of communication. However, in a wide-area wireless network where a base station broadcasts allocation information of downlink burst to a terminal belonging thereto, it is in principle very difficult to perform satisfactory handover within the same wireless network.
As an example of handover to a wireless network of different type, there has been proposed a system using a mobile IP as shown in FIG. 15 (e.g. C. Perkins, “IP Mobility Support (RFC2002)”, [online], October 1996, IETF [searched on 15 Mar., 2006], internet<URL: http://www.ietf.org/rfc/rfc2002.txt>. The basic operations of the mobile IP are as follows.    (i) A physical media, to be connected with LAN1 or LAN2 as a transfer counterpart to which communication is to be transferred, is scanned (in a case of wireless LAN, an effective wireless wave is scanned).    (ii) When a mobile node (MN) 3 enters the transfer counterpart LAN1 or LAN2, a registration request is transmitted to a home agent 6 by way of foreign agents 4, 5. The home agent 6, upon receiving the registration request, transmits an agent advertisement (data including IP address information and the like) to MN3. As a result, registration of the position of MN3 is completed.    (iii) It appears to a correspondence node 7 that the mobile node 3 is located on the home LAN 8. Therefore, the data which the correspondence node 7 wants to send is transmitted toward the home agent 6.    (iv) The home agent 6 encapsulates the data thus sent thereto with a Care of IP Address by using the information of position registration (encapsulation) and transfers the data to the foreign agents 4, 5. A protocol defined exclusively for the mobile IP is used for encapsulation.    (v) The foreign agents 4, 5 dicapsulate the capsule and transfer the data to MN3.    (vi) MN3 transmits to the correspondence node 7 the data MN3 wants to send via the internet 9.Regarding handover between the same wireless communication network, such a handover is carried out by a system in which the uplink and the downlink are simultaneously switched in an unconditional manner.